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| 型号： | LTC4245IG#TRPBF |
| 厂商： | Linear Technology |
| 文件页数： | 25/36页 |
| 文件大小： | 431K |
| 描述： | IC CNTRLR HOT SWAP 36-SSOP |
| 标准包装： | 1,000 |
| 类型： | 热交换控制器 |
| 应用： | CompactPCI? |
| 内部开关： | 无 |
| 电源电压： | 3.3V，5V，12V |
| 工作温度： | -40°C ~ 85°C |
| 安装类型： | 表面贴装 |
| 封装/外壳： | 36-SSOP（0.209"，5.30mm 宽） |
| 供应商设备封装： | 36-SSOP |
| 包装： | 带卷 (TR) |
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LTC4245
25
4245fa
toggles, bit F7 is set to indicate a change of state. If the
GPIO1 alert bit B7 is enabled, this feature can be used to
alert the host system to a change in state of the boards
ejector handles.
Compensating the Active Current Loop
The four active current limit circuits of the LTC4245 are
compensated internally and therefore do not require any
RC network on the gate pins. The internal compensation
should work for most pass transistors. If the gate capaci-
tance is very small then the best method to compensate
the loop is to add a 1nF to 5nF capacitor between the gate
and source of the external MOSFET.
Supply Collapse During Transients
The LTC4245 is designed to ride through supply transients
caused by load steps. If there is a shorted load and the
parasitic inductance back to the supply is signi cant, there
is a chance that the supply could collapse before the active
current limit circuit brings down the gate of the external
MOSFET. In this case the undervoltage lockout circuit,
which has a 3.5約 (5.5約 for V
EEIN
) lter time, turns off
the pass transistors.
Input Overvoltage Transient Protection
Hot-plugging a board into a backplane generates inrush
currents from the backplane power supplies due to the
charging of the plug-in board capacitance. To reduce this
transient current to a safe level, the CPCI Hot Swap speci-
cation restricts the amount of unswitched capacitance
used on the input side of the plug-in board. Each medium
or long power pin connected to the CPCI female connector
on the plug-in board is required to have a 10nF ceramic
bypass capacitor to ground. Bulk capacitors are allowed
on the switched output side of the LTC4245. Some bulk
capacitance is allowed on the Early Power planes, but only
because a current limiting resistor is assumed to decouple
the connector pin from the bulk capacitance (e.g., see
100?to Bus Switch V
DD
in Figure 10).
Disallowing bulk capacitors on the input power pins tends
to create a resonant circuit formed by the inductance of
the backplane power supply trace and the parasitic capaci-
tance of the plug-in board (mainly due to the large power
MOSFET). Upon board insertion, the ringing of this circuit
can exhibit a peak overshoot of 2.5 times the steady-state
voltage (>30V for 12V
IN
).
There are two methods for abating the effects of these
high voltage transients: using voltage limiters to clip the
transient to a safe level and snubber networks. Snubber
networks are series RC networks whose time constants are
experimentally determined based on the boards parasitic
resonance circuits. As a starting point, the capacitors in
these networks are chosen to be 10?to 100?the power
MOSFETs C
OSS
under bias. The series resistor is a value
determined experimentally that ranges from 1?to 50?
depending on the parasitic resonance circuit. Note that
in all LTC4245 circuit schematics, both transient voltage
limiters and snubber networks have been added to the
12V
IN
and V
EEIN
supply rails and should always be used.
Snubber networks are not necessary on the 3V
IN
or the
5V
IN
supply lines since their absolute maximum ratings are
10V. Transient voltage limiters, however, are recommended
as these devices provide large-scale transient protection
for the LTC4245 in the event of abrupt changes in supply
current. All protection networks should be mounted very
close to the LTC4245s supply pins using short lead lengths
to minimize the trace resistance and inductance. A rec-
ommended layout of the 5V and 12V transient protection
devices around the LTC4245 is shown in Figure 18.
Design Example
As a design example, consider a Hot Swap application
with the following power supply requirements:
Table 1. Example Power Supply Requirements
VOLTAGE
SUPPLY
MAXIMUM
LOAD
CURRENT
MAXIMUM
INRUSH dI/dt
LOAD
CAPACITANCE
12V
600mA
150mA/ms
100糉
5V
5A
1.5A/ms
2200糉
3.3V
7A
1.5A/ms
2200糉
12V
300mA
150mA/ms
100糉
1. Select the appropriate values of R
SENSE
for the supplies.
Calculating the value of R
SENSE
is based on I
LOAD(MAX)
and the lower limit for the circuit breaker threshold volt-
age, 擵
SNS(CB)(MIN)
. If a 1% tolerance is assumed for the
sense resistors, then the following values of resistances
should suf ce:
APPLICATIO S I FOR ATIO
U
U
U
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